The European Space Agency’s (ESA) Mars Express has observed the southern part of a partially buried crater that is approximately 270 miles (440 kilometers) wide and informally named Ladon Basin.

The images, near to where Ladon Valles enters this large impact region, reveal a variety of features, most notably the double interconnected impact craters Sigli and Shambe, the basins of which are crisscrossed by extensive fracturing.

This region, imaged April 27 by the high-resolution stereo camera on Mars Express, is of great interest to scientists because it shows significant signs of ancient lakes and rivers.

Both Holden and Eberswalde craters were on the final shortlist of four candidate landing sites for NASA’s Mars Science Laboratory, which is due to land in Gale Crater on August 5/6.

Large-scale overview maps show clear evidence that vast volumes of water once flowed from the southern highlands. This water carved Ladon Valles, eventually flowing into Ladon Basin, an ancient large impact region.

Elliptical craters like this 10-mile-wide (16 kilometers) example are formed when asteroids or comets strike the surface of the planet at a shallow angle.

Scientists have suggested that a fluidized ejecta pattern indicates the presence of subsurface ice that melted during the impact. Subsequent impacts have created a number of smaller craters in the ejecta blanket.

Ladon basin in context. Credit: NASA MGS MOLA Science Team

The interconnected craters Sigli and Shambe are thought to have formed later when an incoming projectile split into two pieces just before impact. The joined craters were then partly filled with sediments at some later epoch.

Ladon Basin perspective view. Credit: ESA/DLR/FU Berlin (G. Neukum)

Deep fractures can be seen within the craters, while in the central and right part of the image smaller craters and more subtle curved fractures appear. These fractures on the basin floor extend beyond the image borders and form concentric patterns. The fractures are believed to have evolved by compaction of the huge sediment loads deposited within the impact basin.

Topographical view. Credit: ESA/DLR/FU Berlin (G. Neukum)

The outflow of Ladon Valles into Ladon Basin is located toward the east of Sigli and Shambe craters, toward the bottom of this image. Here, and in several other parts of the image, lighter-toned layered deposits can be seen. Researchers have detected clay minerals within these deposits, suggesting a relatively long-lasting presence of liquid water in the region’s past.

3D anaglyph view. Credit: ESA/DLR/FU Berlin (G. Neukum)

In addition, winding, valley-like dendritic structures running into the larger basin can be seen above Sigli and Shambe craters, running into the larger impact basin, again indicating flowing water at some distant epoch.